1. Field of the Invention
This invention relates to the field of radio communication apparatus such as mobile telephones, portable telephones and cordless telephones. More specifically, this invention relates to a mobile communication apparatus used as a mobile unit in a cellular radio system.
2. Description of the Related Art
Recently, many kinds of mobile communication systems have been developed and used.
One such system is a cellular radio communication system.
FIG. 5 shows a schematic view of a cellular radio communication system. This system has a control station CS, a plurality of base station, for example, BS1-BS3, and a plurality of mobile units, for example, MU1-MU4. The control station CS is connected to a wire telephone network NW through a wire circuit CL. The base stations BS1-BS3 are connected to the control stations CS through wire circuits CL1-CL3, respectively. The base stations BS1-BS3 form radio zones E1-E3, which are called cells, respectively. The mobile units MU1-MU4 located in the radio zones E1-E3 are connected to the base stations BS1-BS3 through radio channels.
In this kind of cellular system, for example, when mobile units MU1-MU4 move from the radio zone of the base station with which it has had communication, to another radio zone, the operation which is called "handoff control" is carried out. This "handoff control" is executed under the control of the control station CS. In this case, mobile units MU1-MU4 detect a level of "received signal strength" (RSS) of the radio signal which was transmitted from each of base stations. Then mobile units MU1-MU4 transmit the result of the detection to the control station CS. In accordance with the result of this detection, for example, the control station switches the base station so that the mobile station can receive the radio signal which indicates the highest level of the RSS.
On the other hand, mobile units MU1-MU4 have to transmit a radio signal. This radio signal has a certain power strength which is designated by the base station during the radio communication.
Thus, mobile units need to detect their level of the power strength which is called power detection signal "(PDS)". The signals, RSS and PDS are analog signals. Therefore, it is necessary that RSS and PDS should be converted into digital signals. Conventional mobile units utilize a number of A/D converters the same as the number of analog signals which are needed to be converted into digital signals. As a result, the size of circuits increases and the consumption of the electric power of such circuit also increases.